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The presence of CH+ molecules in comets is demonstrated by the attribution of three cometary lines, XX 4231, 4238.5, and 4254.4, to the first transitions of the three branches of the (o, o) band. There is also good evidence for the presence of the (i, o) band of CH~. The CH~ molecules are observed in the head and in part of the tail. The spectral region XX 3560-3600 of cometary spectra is discussed. It seems very probable that some of the features are not due to the ~v = + i sequence of CN and to the (8, o) band of CO~ and that the (o, o) band of OH+ plays some role. But spectrograms of higher resolving-power are required to settle this matter. Additional evidence is given for the presence of the isotopic bands of C2 in cometary spectra; the C12C13 molecule is observed in the three transitions (i, o), (2, i), and (~, ~). The problems of molecular formation, dissociation, and ionization are quite different in interstellar space and in comets. In interstellar space the main source of molecular formation is by two-body en- counters with simultaneous emission of radiation. In comets the diatomic molecules result from the liberation of occluded gases and from the photodissociation of more complex molecules. The dissociation equilibrium of CH+ in interstellar space is discussed. Account has to be taken of the fact that practically all the CH~ molecules are on the lowest rotational level. Thus the photodissociation may arise oniy from this lowest level, whereas the formation by capture may lead to any excited level. The corresponding correction factor has a value of the order of io3. The computed abundance of CH+ is in agreement with the observations. The ionization equilibrium of CH~ and CH is considered. It is suggested that the electron density in the regions of molecular absorption is about io' electron per cubic centimeter. The dissociation and ionization equilibrium of CN cannot yet be discussed because of the absence of essentia' physical data, especially the ionization potential of CN. The abundance of C2 molecules must be much lower than that of CN or CH; moreover each (v', o) band of the interstellar C2 mblecules would possess a large number of absorption lines, instead of three or less as in the case of CH, CH+, and CN. From these considerations it is concluded that interstellar lines of C2 must be very much weaker than those of CH, CH~, and CN